Because there is a continuing requirement to increase the integration density of integrated circuits, new techniques for creating a pattern on a surface such as the surface of a semiconductor wafer have been developed. One such technique utilizes an electron-beam lithography system in which electrons are directed onto the surface of a semiconductor wafer to expose an electron sensitive resist coating on the surface of the semiconductor wafer. After exposure and development, the resulting resist pattern is used as a template to effect patterned modifications on or to the underlying semiconductor material.
The classic electron-beam lithography system is the "probe-forming" system in which a narrow beam of electrons that is the image of the electron source and having a Gaussian distribution is scanned over the surface to be exposed. The narrow beam is scanned a pixel at a time, the pixel being defined as the full width at half height of the beam intensity distribution. These systems can have the highest spatial resolution, but also have the lowest throughput of all electron-beam lithography systems due to the serial exposure of patterns one pixel at a time. One advantage of systems that serially expose patterns is that corrections can be applied dynamically, pixel by pixel, to compensate for aberrations of the electron lenses and deflection units in the system. Another advantage is that if the pixel represents the smallest feature of the desired pattern, a pattern of any arbitrary complexity can be created with this type of probe. A further advantage is that proximity corrections are also easily made with this type of probe.
An increase in throughput can be achieved by producing a larger spot on the wafer, adjustable in size and shape, so that it is equal to or greater than the minimum feature size of the circuit. Electron-beam lithography systems with this feature create a shaped spot on the wafer by generating an image of apertures or other objects illuminated by the source, that is, not an image of the source itself. In these types of electron-beam lithography systems, the image is electronically variable in size and adjustable to compose a pattern feature with serial exposures projecting up to several hundred pixels in parallel.
An increase in throughput can also be achieved by providing a mask (reticle) through which a beam of electrons is directed and electronically focused onto the surface of the semiconductor wafer. As is known in the semiconductor manufacturing art, the reticle has a pattern formed thereon that describes the features of the circuit or structure that is to be formed on the semiconductor wafer. Because the size of the reticle far exceeds the field of view of the electronic lenses that are available, it is necessary to direct the electron beam onto a portion of the reticle and then move (step or scan) the electron beam sequentially to other portions of the reticle. To facilitate this procedure, the reticle is divided into subfields and each subfield is illuminated one at a time by sequentially stepping or scanning the electron beam from one subfield to the next until the pattern on the reticle has been completely transferred by an electronic projection lens system onto the surface of the semiconductor wafer. Alternatively, the reticle may be moved so that other portions of the reticle are sequentially illuminated by the electron beam.
Because of the increase in the integration density of the integrated circuits on semiconductor wafers, the positional accuracy of the electronic systems for transferring the pattern from the reticle to the wafer has approached and has exceeded the practical limitations of available electronic equipment.
Accordingly, there is a need for an electronic system that utilizes available electronic equipment to accurately position the electron beam on the semiconductor wafer that is cost-effective and that does not degrade throughput of the lithography system.